1. Field of the Invention
The present invention relates to a method for producing bipolar transistors which are isolated from each other and from other elements by an insulating layer and which have emitter regions that are in contact with the insulating layer, and further relates to bipolar transistors produced by such a method.
2. Description of the Prior Art
A bipolar transistor is used as an element of a semiconductor device, e.g. IC or LSI. As such a bipolar transistor, there is known a so-called "walled emitter type bipolar transistor," illustrated in FIGS. 1A and 1B. FIG. 1B is a sectional view taken along line A--A' of FIG. 1A. This type of transistor is isolated by a field insulating layer 2, which comprises silicon dioxide (SiO.sub.2) and is formed on a semiconductor body 1, and has an emitter region 3 which is in contact with the field insulating layer 2 and lies within a base region 4. A portion of the field insulating layer 2 encroaches on the semiconductor body 1 to demarcate an area for an element, e.g. a transistor. In this case, the collector region of the transistor is the semiconductor body 1, and is connected to a collector electrode (not illustrated in FIGS. 1A and 1B). The transistor is provided with an emitter electrode 8, a base electrode 8', and an insulating layer 6.
The above-mentioned bipolar transistor is produced in the following manner. The starting material is the semiconductor body 1 (FIG. 2A) of a first conductivity type (e.g. n-type), consisting of a single-crystalline silicon substrate and a silicon epitaxial layer grown on the substrate. In case of a bipolar integrated circuit device, the conductivity type of the substrate is generally p-type, and that of the epitaxial layer formed on the substrate is n-type. Circuit elements, e.g. bipolar transistors or resistors, of the integrated circuit device are formed in the epitaxial layer. Furthermore, isolation of the elements from each other is performed by forming an insulating material, a p-type region, or a V-groove, which extends from the surface of the epitaxial layer to the substrate. If desired, prior to the growth of the epitaxial layer, a so-called buried collector layer (n.sup.+ -type) is formed in the usual manner. The semiconductor body 1 is selectively oxidized to form a field insulating oxide (SiO.sub.2) layer 2 (FIG. 2A), isolating elements of the semiconductor device from each other. Impurities of a second conductivity type opposite to the first conductivity type (e.g. boron ions of p-type) are introduced into the semiconductor body 1 by an impurity diffusion process to form a base region 4 (FIG. 2B). The surface of the semiconductor body 1 is oxidized simultaneously or after the impurity diffusion to form an insulating layer, i.e. a silicon dioxide (SiO.sub.2) layer 6 (FIG. 2B). Then, the silicon dioxide layer 6 is selectively etched by a conventional photo-etching process to open a hole 7 (FIG. 2C) of which a part of the side is formed by the insulating layer 2. It is preferable to etch the end part of the insulating layer 2 simultaneously with the etching of the silicon dioxide layer 6, as illustrated in FIG. 2C. Impurities of the first conductivity type, e.g. phosphorus ions, are introduced into the base region 4 of the semiconductor body 1 through the hole 7 to form an emitter region 3 (FIG. 2C) which is in contact with the insulating layer 2. Then, a hole for a base electrode is opened in the silicon dioxide layer 6 by a conventional photo-etching process. Finally, an emitter electrode 8 and a base electrode 8' made of metal, e.g. aluminum, are formed to produce the bipolar transistor, as illustrated in FIG. 1B. In this case, a collector electrode (not illustrated) is formed at a suitable position simultaneously with the formation of the emitter and base electrodes.
However, the above-mentioned bipolar transistor has problems, in that the breakdown voltage between the collector and the emitter is low, and a short-circuit between the collector and the emitter is occasionally generated. These problems are caused by formation of a channel having a conductivity type opposite to the conductivity type of the base region, e.g. an n-type channel, at an interface 5 (FIG. 1B) between the base region 4 and the insulating layer 2.
When impurities are introduced into a semiconductor body by a conventional impurity diffusion process, the concentration of impurities is the semiconductor body is highest at the surface thereof, and gradually decreases with an increase in the distance from the surface. It is well known by persons skilled in the art that an n-type channel can be induced in a p-type region adjoining a silicon dioxide layer, when the concentration of impurities of the p-type region is 10.sup.16 .about.10.sup.17 atoms/cm.sup.3. When a walled emitter type bipolar transistor is produced in the above-mentioned manner, the concentration of impurities of the base region 4 at the interface 5 (FIG. 1B) between the base region 4 and insulating layer 2 becomes 10.sup.16 .about.10.sup.17 atoms/cm.sup.3, so that the interface 5, i.e. the side surface of the base region 4, is easily changed to an n-type from a p-type. Namely, the n-type channel is easily formed at the side surface of the p-type base region 4.